Britain's longest-serving monarch died Thursday after several months of declining health.
All Queen Elizabeth's family was at the Balmoral Royal estate in Scotland at the time of her passing, or in travel to be there.
"The death of my beloved Mother, Her Majesty The Queen, is a moment of the greatest sadness for me and all members of my family," the new king, her eldest son Charles, said. He has chosen the name King Charles III.
"We mourn profoundly the passing of a cherished Sovereign and a much-loved mother. I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world," the 73-year-old said in a statement.
Buckingham Palace said Charles and his wife Camilla, now the Queen Consort, would remain at Balmoral Castle, before returning to London on Friday.
"The death of Her Majesty the Queen is a huge shock to the nation and to the world," new Prime Minister LIz Truss said outside her Downing Street office where the flag, like those at royal palaces and government buildings across London, were lowered to half-mast. Tryuss met the Queen at Balmoral only two days ago.
The queen -- an instantly recognizable figure to billions of people across the world -- was celebrating Platinum Jubilee year on the throne in 2022. She ascended after the death of her father King George VI on Feb. 6, 1952, when she was just 25. His death took place while she was on a visit to Africa.
Before Thursday's announcement, notes were passed in the House of Commons to Prime Minister Liz Truss, her ministers and opposition leaders, prompting them to leave the chamber.
"My thoughts -- and the thoughts of people across our United Kingdom -- are with Her Majesty The Queen and her family at this time," Truss added, echoed by leaders in Scotland, Wales and Northern Ireland.
One photograph of the queen greeting Truss at Balmoral on Tuesday had already sparked alarm, showing a deep purple bruise on the monarch's right hand.
On Wednesday, a day after Truss's appointment as the 15th prime minister of her reign, the queen pulled out of a planned meeting with her council of political advisers, after being told to rest.
Her passing comes after she has looked visibly frailer in recent months, and a succession of withdrawals from public engagements.
She had taken to walking with the help of a stick and was also seen earlier this year at the Chelsea Flower Show touring the site in a motorized buggy.
As well as the United Kingdom, the queen was also head of state in 14 Commonwealth countries around the world, including Canada, Australia and New Zealand.
She further headed the Commonwealth grouping, which comprises 56 nations and takes in more than a quarter of humanity.
For most of her subjects, she was the only monarch they have ever known, featuring on stamps, banknotes and coins, and immortalized in popular culture.
But Britons were forced to face up to the reality that her reign in her twilight years, when her husband of 73 years, Prince Philip, died in April 2021, just weeks shy of his 100th birthday.
"She is just part of our lives... She's been the queen for all our lives really" said Maureen Barnett, 66, outside Buckingham Palace as news filtered through.
This report was prepared with information from Reuters and Agence France-Presse